When a blood pressure is measured, a cuff functioning as an ischemic band including a fluid bladder for blood pressure measurement is wound around a part of a living body and fixed, and then a pressure of the fluid bladder is increased or reduced. A technique, in which a change in volume of a compressed blood vessel is captured as a change in amplitude of a fluctuation in cuff pressure by increasing or decreasing the pressure of the cuff wound around a part of the living body and the blood pressure is computed, is called an oscillometric method.
In an electronic blood-pressure meter in which the oscillometric method is adopted, when the state of the cuff pressure is changed, the amplitude of the fluctuation in cuff pressure generated by the change in volume of the blood vessel is also changed. Even in the same cuff pressure, when the volume of the cuff is changed, the amplitude of the fluctuation in cuff pressure is changed. That is, depending on softness of a measurement region (such as upper arm) to be measured, a size (circumferential length) of the measurement region, and a cuff winding state, the amplitude of a pressure pulse wave is changed when the cuff volume is changed although an artery is compressed with the same cuff pressure. Specifically, the pressure pulse wave is decreased as the cuff volume is increased, while the pressure pulse wave is increased as the cuff volume is decreased. That is, the amplitude of the pressure pulse wave is changed according to measurement states except for the information on the blood pressure of the living body, and the large cuff volume is required to increase the cuff pressure to the same level when the measurement region has the large size or when the living body is soft. Therefore, the pressure pulse wave amplitude used to compute the blood pressure is changed depending on the measurement state, which influences measurement accuracy. In order to suppress an error caused by the measurement state except for the information on the blood pressure of the living body, Japanese Patent No. 3113737 (hereinafter, referred to as Patent Document 1) discloses an electronic blood-pressure meter in which the a characteristic of the change in cuff volume to the cuff pressure is previously provided to convert a signal of the change in cuff pressure into the change in cuff volume and a blood pressure value is measured using the converted cuff volume.
When the cuff is fixed with an insufficient force, the fluid bladder is inflated outside without compressing the blood vessel during pressurization, and the fluid bladder does not effectively fulfill an original role in pressing and closing the blood vessel, which results in deterioration of accuracy of blood pressure measurement. When the winding state is changed during the pressurization, accuracy of blood pressure estimation performed during the pressurization is largely deteriorated. Accordingly, after the cuff is wound around the living body, it is necessary that the winding state be securely fixed to suppress an error.
The following configurations fixing the cuff are well known. There is well known a configuration in which the cuff winding state is maintained by a surface fastener. In a blood pressure measuring apparatus in which the cuff is automatically wound around the living body to measure the blood pressure, for example, Japanese Patent Laying-Open No. 2004-215847 (hereinafter, referred to as Patent Document 2) discloses a lock mechanism engaging a slide unit to become immobile at a wind end position in a mechanism in which one end of the cuff is pulled by a slide mechanism to wind the cuff around a part of the living body, and Japanese Patent Laying-Open No. 6-14889 (hereinafter, referred to as Patent Document 3) discloses a lock engaged with a wind-up mechanism at the end of winding to prevent reverse rotation in a mechanism in which the cuff is wound around a part of the living body by winding up a rope connected to the cuff.
Japanese Patent Laying-Open No. 2005-230175 (hereinafter, referred to as Patent Document 4), previously applied by the inventor, discloses a blood pressure measuring apparatus having a configuration in which, unlike the configuration of the usual blood pressure measuring apparatus, two fluid bladders independently provided with a curler interposed therebetween are used for winding of the cuff and the blood pressure measurement.
As to the configuration of the blood pressure measuring apparatus in which the two fluid bladders independently provided with a curler interposed therebetween are used for winding of the cuff and the blood pressure measurement, Japanese patent Laying-Open No. 11-309119 (hereinafter, referred to as Patent Document 5) and Japanese Patent Laying-Open No. 11-318835 (hereinafter, referred to as Patent Document 6) disclose a configuration in which pressing means for supplying a predetermined amount of fluid to a compressing fluid bladder compressing a human body and pressing the compressing fluid bladder against the living body is provided. Japanese Patent Laying-Open No. 5-269089 (hereinafter, referred to as Patent Document 7) discloses a configuration in which a small inner cuff compressing an artery is filled with a low-viscosity conduction solution and the inner cuff is pressed against the human body by an outer cuff located outside the inner cuff.
A measuring operation shown in FIG. 12 is performed in the blood pressure measuring apparatus having the configuration disclosed in Patent Document 4. Referring to FIG. 12, initialization is performed in Step S1, and the measuring air bladder functioning as the measuring fluid bladder is compressed against the blood pressure measuring part through the curler by supplying air to a compressing and fixing bag functioning as the compressing fluid bladder in Step S2. When a pressure of the measuring air bladder reaches a predetermined pressure, it is determined that the measuring air bladder is wound around the blood pressure measuring part, and the pressurization is ended in Step S3. In Step S4, the living body is compressed to pressurize the measuring air bladder functioning as the air bladder for measuring the blood pressure to a pressure enough to press and close the blood vessel. In Step S5, an artery pressure pulse wave and a pressure value are detected while the pressure is reduced. In Step S6, the blood pressure is computed based on the artery pressure pulse wave and the pressure value. The measurement result is displayed in Step S7, and the air in the compressing and fixing air bladder and the measuring air bladder is vented to release the compression of the living body in Step S8.
Referring to FIGS. 13 to 16, the pressurization and depressurization operations of the compressing and fixing air bladder and the measuring air bladder during the measuring operation will be described in detail.
FIG. 13 is a schematic view for illustrating the pressurization and depressurization operations of the compressing and fixing air bladder and the measuring air bladder in Steps S2 and S3. Referring to FIG. 13, at the start of the measurement, the compressing and fixing air bladder is in the non-pressurized state in which the compressing and fixing air bladder is released to an atmospheric air and the compressing and fixing air bladder is in the compressive state. Then, a predetermined amount of air is supplied to the measuring air bladder, and the compressing and fixing air bladder is preliminarily pressurized.
FIG. 14 is a schematic view for illustrating the pressurization and depressurization operations of the compressing and the fixing air bladder and measuring air bladder in Step S4. Referring to FIG. 14, the internal pressure of the measuring air bladder preliminarily pressurized in Steps S2 and S3 and the change in internal pressure reach predetermined values, the supply of the air to the compressing and fixing air bladder is ended, and the air is supplied to the measuring air bladder to pressurize the measuring air bladder in Step S4. When the air is supplied to the measuring air bladder to pressurize the measuring air bladder, the measuring air bladder is inflated inside in a radial direction because an outer circumference is fixed by a housing, and the curler is pressed in an inner diameter direction. Therefore, the measuring air bladder provided inside the curler is pressed against the measurement region.
FIG. 15 is a schematic view for illustrating the pressurization and depressurization operations of the measuring air bladder in Step S5. Referring to FIG. 15, when the air is supplied to the measuring air bladder to reach the internal pressure enough to press and close the artery in Step S4, the supply of the air is ended, and the air of the measuring air bladder is discharged to depressurize in Step S5. When the pressure is reduced, the pressing force acting on the artery is weakened. In Step S5, the internal pressure of the measuring air bladder is measured at that time to detect the artery pressure pulse wave.
FIG. 16 is a view showing fluctuations in internal pressures of the compressing and fixing air bladder and measuring air bladder in first to fourth procedures. In FIG. 16, the first procedure (I) is a procedure for preliminarily pressurizing the measuring air bladder, the second procedure (II) is a procedure for pressurizing the compressing and fixing air bladder in Step S2, the third procedure (III) is a procedure for pressurizing the measuring air bladder in Step S4, and the fourth procedure (IV) is a procedure for depressurizing the measuring air bladder in Step S5.
Referring to FIG. 16, in the first procedure, because the compressing and fixing air bladder is in the non-compressive state, the internal pressure of the compressing and fixing air bladder becomes substantially zero, and a predetermined amount of air is preliminarily supplied to the measuring air bladder to detect winding. When the predetermined amount of air is preliminarily supplied to the measuring air bladder, the supply of the air is ended, and then the pressurization of the compressing and fixing air bladder is started as the second procedure.
In the second procedure, the internal pressure of the measuring air bladder and the change in internal pressure are monitored, and the compressing and fixing air bladder is pressurized until the internal pressure of the measuring air bladder and the change in internal pressure reach predetermined values.
Then, in the third procedure, the measuring air bladder is pressurized while the internal pressure of the compressing and fixing air bladder is maintained. When the measuring air bladder reaches the pressure enough to press and close the artery, the pressure of the measuring air bladder is reduced in the fourth procedure.    Patent Document 1: Japanese Patent No. 3113737    Patent Document 2: Japanese Patent Laying-Open No. 2004-215847    Patent Document 3: Japanese Patent Laying-Open No. 6-14889    Patent Document 4: Japanese Patent Laying-Open No. 2005-230175    Patent Document 5: Japanese Patent Laying-Open No. 11-309119    Patent Document 6: Japanese Patent Laying-Open No. 11-318835    Patent Document 7: Japanese Patent Laying-Open No. 5-269089